The invention relates to an arrangement for varying the tape tension in a magnetic-tape apparatus, comprising a pivotably supported sensing lever whose free end is pressed against the magnetic tape under the influence of a spring which acts upon the sensing lever and which sensing lever, upon a deflection from a neutral position, is reset to its neutral position by a control device by means of a variation of the tape tension.
In a known arrangement of the type defined in the opening paragraph the tape forces, on the one hand, and the force exerted on the sensing lever by the spring, on the other hand, are in equilibrium in the neutral position of the sensing lever. When this equilibrium is disturbed, for example by an increase in the spring force, the sensing lever will deflect. This deflection is detected by a position sensor and transmitted to a control device. The motor current for the winder is then increased to such an extent that the increased tape tension causes the sensing lever to resume its neutral position. Thus, control is effected so as to maintain an equilibrium between, on the one hand, the force acting on the end of the sensing lever and, on the other hand, the tape forces.
If the magnitude of the tape forces is to be changed, for example for operation in the play mode or the shuffle mode, the spring force has to be changed. This is effected in known manner in that the point of application of the spring on the sensing lever is transferred to another position. When the point of application of the spring is thus transferred the resulting deflection of the sensing lever out of its neutral position is cancelled by the control system, i.e. the sensing lever is reset to its neutral position by a correspondingly increased or reduced tape tension.
The change in tape tension, i.e. a reduction of the tape tension, is particularly important in a so-called "stand-by" mode of a magnetic-tape apparatus. In such a mode, the magnetic tape is not driven but the magnetic heads of the scanner rotate and are thus in contact with the same portion of the tape for a longer time. This may lead to considerable damage to the tape. Therefore, it is useful to reduce the tape tension in such a stand-by mode. In the above-mentioned known construction this is effected by transferring the point of application of the spring on the sensing lever by means of a lead screw and a self-locking gear-wheel. Since it is not possible to predict when the stop command for the magnetic tape will be followed by a subsequent command for another mode of operation the lead screw has to be actuated very frequently and often unnecessarily if the next command occurs after a very short time. This means an increased wear. Moreover, the starting time for the next command increases because the lead screw should first transfer the spring end to another position.
DE-GM 17 66 285 describes a deck for magnetic-tape apparatuses comprising a sensing-lever arrangement for maintaining the tension of the magnetic tape constant. Therein, sensing levers which engage with the circumference of a sound-carrier spool are coupled to an actuating element for control devices for winding motors. This arrangement has been constructed in such a manner that the sensing levers are interconnected by a lever system by which they are jointly pivotable into their operating position or into a rest position upon actuation of a mode selection switch, if desired by relay control. Thus, the sensing levers are operative only in the "recording" or the "playback" mode. This reduces magnetic tape wear and simplifies mounting of the reel hubs.
A similar construction, which also serves to reduce the tape tension when the magnetic tape is stationary, is known from JP-2-246039. It comprises a pivotable lever which is spring-loaded and whose free end engages with the magnetic tape. The tape tension can then be changed by means of a first or a second sensor.